The evidence of whether marijuana smoking causes lung cancer seems to be somewhat mixed, with some studies finding evidence that it does (and this really should not be a surprise if it does), and others finding no evidence for it. Here is a recent longitudinal study indicating that there is reason for concern:

As the paper explains, some case-control studies do not show a reason for concern, while others do show an association. Especially in light of the very similar nature of marijuana smoke and tobacco smoke, and that pot smokers do not just inhale, but hold it in their lungs, it would be a bit surprising if there was not an association.